1. Field of the Invention
This invention relates to the preparation of polyorganosiloxane foams. More particularly, this invention relates to polyorganosiloxane compositions that are converted to useful cured foams when the ingredients of the composition, including a blowing agent, are combined and dispensed into an area under atmospheric pressure at a temperature of about 0.degree. C. or higher.
2. Background Information
A number of different methods for generating curable polyorganosiloxane foams are disclosed in the prior art. One of these methods employs the reaction of silicon-bonded hydrogen atoms with a source of hydroxyl groups such as water, alcohols or polyorganosiloxanes containing silicon-bonded hydroxyl groups. This reaction generates hydrogen gas that functions as the blowing agent. The use of internally generated hydrogen as a blowing agent for polyorganosiloxane foams is disclosed in U.S. Pat. No. 3,923,705, issued to Smith on Dec. 2, 1975, in U.S. Pat. No. 4,026,842, issued to Lee and Ronk on May 31, 1977 and in U.S. Pat. No. 4,026,844, issued to Kittle and Ronk on May 31, 1977.
Production of foams by stirring air or other gas into a curable polyorganosiloxane composition containing silicon-bonded vinyl radicals and silicon-bonded hydrogen atoms is disclosed in U.S. Pat. No. 4,368,279, issued to Modic and Boudreau, on Jan. 11, 1983. In accordance with the method described in this patent, the polyorganosiloxane composition is mixed in the presence of the gaseous blowing agent and then placed in a vacuum chamber and maintained under a vacuum of at least 600 mm of mercury until the foam cures sufficiently to become self-supporting, which requires at least five minutes. Patentees teach that collapse of the foam occurs in the absence of vacuum, and that their method is limited to producing foamed slab stock in a factory, and cannot be applied to producing foam in a conduit, between inner and outer structural walls or other locations where a foam is generated at the installation site.
The advantage of packaging in a single portable container all of the ingredients, including blowing agent required to produce a polyorganosiloxane foam has been recognized. U.S. Pat. No. 4,229,548, issued on Oct. 21, 1980 to Sattleger et al. discloses a 2-compartment aerosol type container for storing and dispensing a foamable polyorganosiloxane composition. The container consists of a flexible walled inner compartment containing a foamable, room temperature vulcanizable (RTV) composition that includes a hydroxy-endblocked polydiorganosiloxane, a curing agent and, optionally, a gaseous blowing agent and an outer compartment containing a moisture free inert gas under a pressure of from 0.2 to 3.0 megapascals. The container is equipped with a valve through which the foamable composition is dispensed under the pressure of the gas confined in the outer compartment of the container. The formation of polyorganosiloxane foams by dispensing a one-part moisture curable RTV polyorganosiloxane composition stored under pressure in a 2-compartment container is also taught in German published application Nos. 2,909,443 and 2,911,971.
Foams produced by dispensing RTV polyorganosiloxane compositions, including a blowing agent and/or a propellant, from pressurized containers, such as aerosol cans, are typically of relatively poor quality and characterized by average cell sizes larger than 2 mm., densities from 0.48 to 0.81 g./cc and relatively low foam height due to drainage of uncured or partially cured liquid from the cellular structure of the foam during the curing process. The need to minimize collapse of partially cured foams by the use of vacuum, by heating to accelerate curing, by the addition of fillers or other means requiring additional processing steps may more than offset the advantages achieved by using foamable compositions packaged in portable pressurized containers such as aerosol cans.
Various additives have been used to reduce the density of polyorganosiloxane foams. One such additive is a resinous organosiloxane copolymer containing repeating units of the formulae R.sub.3 SiO.sub.1/2 and SiO.sub.4/2, where R represents alkyl, aryl, aralkyl, alkaryl, cycloalkyl, vinyl, allyl or fluoroalkyl and the molar ratio of R.sub.3 SiO.sub.1/2 to SiO.sub.4/2 units is from 0.25 to about 0.8:1. This type of additive is disclosed in U.S. Pat. No. 4,418,157, which issued to Modic on Nov. 29, 1983.
Kim, Lee and Ronk in U.S. Pat. No. 4,026,845, issued on May 31, 1977, teach the addition of fluorinated surfactants to reduce the density of foams generated by the hydrogen produced during the reaction of silicon-bonded hydroxyl groups with silicon-bonded hydrogen atoms in the presence of a platinum-containing catalyst. It has now been found that these fluorinated surfactants will not stabilize the cellular structure of partially cured polyorganosiloxane foams dispensed from pressurized containers in the absence of large amounts of solid fillers.
An objective of this invention is to provide foamable polyorganosiloxane compositions that are curable by a hydrosilation reaction, can be packaged in pressurized containers and when dispensed from these containers form useful foams. A second objective of this invention is to provide one-part foamable compositions that cure in the presence of microwave radiation.